The invention relates to a leak detection device for the measurement of the differential pressure between a test chamber and a reference chamber, as well as to a method for the measurement of a corresponding differential pressure.
It is known to measure the pressure difference between the internal pressure in a specimen and the internal pressure in a test chamber. The specimen may be a food package. Prior to the differential pressure measurement, the specimen and the reference chamber are pressurized with the same overpressure. In case of a leaky specimen a pressure difference develops between the specimen and the reference chamber during the differential pressure measurement, since the overpressure escapes from the specimen through the leak, whereas the overpressure remains in the hermetically sealed reference chamber. The measured pressure difference serves as a measure for the leakage rate of the specimen. The advantage of differential pressure measurement is that the pressure sensor does not have to be designed for the pressures prevailing in the specimen and the reference chamber. Pressures of 10 bar may e.g. prevail in the specimen and the reference chamber, whereas the pressure sensor is designed only for a range up to 100 mbar. It is another advantage that the temperature influences on the total pressure in the specimen act analogously on the pressure in the reference chamber and thus do not influence the differential pressure.
EP 0 313 678 B1 and EP 0 763 722 B1, respectively, describe an arrangement of a test chamber holding a specimen and a reference pressure system adapted to be separated from the test chamber by means of a check valve, wherein the differential pressure between the pressure in the test chamber and the pressure in the reference pressure system is measured using a differential pressure sensor. The test chamber and the reference pressure system are each formed with rigid walls, the test chamber being designed to receive the specimen and the reference pressure system being designed as a piece of conduit. Further, a reference chamber is described which serves to pressurize the test chamber with a predefined reference pressure.
Further, film chambers for leakage tests on non-rigid specimens are known. In DE 19722262 A1 it is described to form a test chamber of a film so as to minimize the test space, i.e. the space remaining between the specimen and the test chamber. For a testing of the tightness of a non-rigid specimen, e.g. a packaging bag, in a film chamber without using test gas, the film chamber containing the specimen is evacuated and, thereafter, the rise in total pressure inside the film chamber is measured in the region outside the specimen. The film surrounding the specimen further prevents a bursting of the specimen.
For a sufficiently precise measurement of the pressure increase in the film chamber, the total pressure inside the film chamber has to be evacuated to a sufficiently low value before. A pressure (vacuum) of less than 100 mbar is necessary to achieve a sufficient measuring accuracy with total pressure measuring elements. Typically, pressures of less than 10 mbar and preferably less than 1 mbar are reached. With pressures that low, gas proportions that are degassed from the film wall of the film chamber and from a nonwoven typically arranged between the specimen and the film wall, cause an increase in pressure.